Direct-view and projection display systems conventionally employ kinescopes as display devices. During normal operation of a kinescope, scanning circuitry deflects and electron beam to produce a relatively large area raster on the face plate of the kinescope and video modulation of the beam produces a visible picture by activating phosphors deposited on the face plate. The beam energy in normal operation is distributed over the whole area of the kinescope face plate. If scan loss should occur, the beam energy may be concentrated on a relatively small area of the face plate and this high concentration of beam energy may produce permanent damage to the phosphor, a so-called "spot burn". Scan loss may occur when the display device is turned on under "hot start" conditions (i.e., when the receiver is rapidly cycled between on and off modes) or it may occur, for example, when a component failure occurs.
Occurrence of scan loss conditions is also a concern when a kinescope is turned off. When the power supply to a kinescope is removed, the cathode continues to emit electrons until it has sufficiently cooled off. As a result, the cathode continues to emit a decaying electron beam for a definite period of time after the deflection voltages have been cut off. In order to prevent spot burns on the face plate during this period, it is desirable to maintain a sufficient bias voltage between the cathode and the control grid to prevent the decaying electron beam from illuminating the face plate without deflection voltages.
During normal operation, a voltage difference is maintained between the cathode and the control grid. A large voltage difference results in low illumination levels on the display screen and a low voltage difference results in high illumination levels. For example, a typical cathode voltage may be about 180 volts and a typical grid voltage may be in a range of about ten to twenty volts or so, and the cathode voltage is modulated to reduce the voltage difference to change the luminance level. When the display is turned off, the cathode and grid voltage difference goes toward zero, and if deflection voltages are not present, the zero cathode bias causes the electron beam to be concentrated on a very small area of the display screen.
One method of preventing spot burn during kinescope turn-off is the so-called "grid kick" method. In such a method, a charge storage device is coupled to a control grid of the cathode ray tube and further coupled through a switch device to a voltage supply. The charge storage device is charged through the switch device by the voltage supply when a control signal coupled to a control input of the switch device indicates that a deflection signal is present. The switch device decouples the supply voltage from the charge storage device when the control signal indicates that the deflection signal is not present and places a negative blanking voltage on the control grid. In this manner, a sufficient voltage difference is maintained between the cathode and the control grid when the cathode voltage collapses, and thus the display screen remains blanked. Such a method is described, for example, in U.S. Pat. No. 5,089,754 entitled "Protection Circuit for a Cathode Ray Tube" which issued Feb. 18, 1992 to John B. George. Another example of a "grid kick" type of kinescope protection circuit is described by Gurley et al. in U.S. Pat. No. 5,043,639 entitled "Video Display Apparatus With Kinescope Spot Burn Protection Circuit" which issued Aug. 27, 1991. The Gurley et al. circuit is similar to that of George but utilizes a passive charging source and a switching device coupled between a plate of the capacitor and ground.
Although the above-described grid kick circuits employ only a single switching device, they utilize relatively complex additional bias circuitry as compared, for example, to arrangements employing multiple switching devices as, for example, the grid kick circuitry employed in the model CTC-195 Color Television Receiver manufactured by Thomson Consumer Electronics, Inc. FIG. 1 herein is a schematic diagram of such a grid kick circuit in a television receiver.
Advantageously, the use of a pair of switching devices in the grid kick circuit of the CTC-195 receiver provides active pull-down of the grid kick capacitor giving a rapid reduction in grid voltage and additionally provides an actively regulated DC bias voltage for the grid during normal operation of the receiver.
The advantageous features of the CTC-195 grid kick circuit of active pull down of the kick capacitor when beam blanking is required and active regulation of the grid bias when blanking is not required might lead one to believe that no further improvement in the circuitry would be necessary.
However, in accordance with an aspect of the present invention, it has been found that under certain grid kick circuit applications, it would be desirable to further improve the performance of the grid kick circuit. Specifically, it has been found that in applications where the supply voltage to the grid kick circuit is relatively low, that a need exists for improving the charging efficiency of the grid kick circuitry. As discussed in detail later with regard to the prior art FIG. 1 circuit, the charging efficiency of the prior art circuit is only about 60% expressed in terms of the voltage stored on the capacitor as a percentage of the value of the high voltage supply. While this charging efficiency is perfectly adequate where supply voltages in the order of 250 volts is available, it may result in marginal beam cut-off performance in applications where the high voltage supply is appreciably less than 250 volts.
It is, therefore, desirable to provide a grid kick circuit having improved charging efficiency for the kick capacitor.